In many technical fields, particularly in the field of microlithography, it is desirable, among other things, to subject bodies and the surfaces thereof to a rigorous optical inspection in order, for example, to be able to assess the quality of a production process and, where need be, to be able to intervene in a correcting manner should the inspection determine that predetermined quality criteria are not being met. Naturally, the same, or even more stringent, characteristics should be placed here on the precision of the imaging device used for the inspection when compared to the devices used in the production process of the body to be inspected.
In this context, the capability of the imaging device used for the inspection to process light of different wavelengths with the smallest possible aberrations is of particular importance in order to ensure a broad field of application for the imaging device. Thus, particularly in the context of production methods comprising an optical process, it is desirable or advantageous if the imaging device used for the inspection can process, with minimized aberrations, the wavelength range which is also used during the optical process. By way of example, this may be the wavelength range from 193 nm (the so-called VUV-range) to 436 nm (the so-called Hg g-line).
In this wavelength range, only a few optical materials still have a sufficient transparency, and so the systems are predominantly built from synthetic quartz glass (SiO2) and fluorspar (CaF2).
Here, the chromatic aberrations, i.e. the aberrations which are dependent on the wavelength of the light, are problematic. By way of example, if an imaging device with refractive optical elements (such as lens elements or the like) is used for the inspection, the aberrations of the imaging device are generally only minimized for a comparatively narrow wavelength range with a justifiable amount of outlay. A so-called achromatization of such a dioptric imaging device, i.e. of an imaging device only comprising refractive optical elements, that is to say an elimination of such chromatic aberrations, can hardly still be done with justifiable outlay over a broadband wavelength range (such as the aforementioned wavelength range).
Therefore, use is often made of so-called catadioptric imaging devices, which, in addition to refractive optical elements, also comprise reflective optical elements which are more expedient in respect of chromatic aberrations. By way of example, such catadioptric systems are known from U.S. Pat. No. 5,031,976 (Shafer), U.S. Pat. No. 5,717,518 (Shafer et al.), US 2004/0027688 A1 (Lange) and U.S. Pat. No. 7,136,159 B2 (Tsai et al.), the entire disclosures of which are respectively included herein by reference.
In the context of wafer inspection, US 2004/0027688 A1 (Lange) has disclosed a high aperture (numerical aperture NA greater than 0.9), strongly magnifying catadioptric microscope objective, which images an object at infinity, wherein the image is broadband, i.e. corrected over a large range of wavelengths. Here, in one variant, an optical element group adjoins the microscope objective, which optical element group initially generates an intermediate image before the light is subsequently collimated again and fed to a zoom group with positive refractive power. A problem in this zoom system is that, in the case of a compact design, only comparatively small maximum focal lengths and therefore comparatively small maximum magnifications (and consequently only a comparatively small extension of the imaging scale) can be realized.
In the context of wafer inspection, U.S. Pat. No. 7,136,159 B2 (Tsai et al.) describes a high aperture (numerical aperture NA up to 0.99), strongly magnifying catadioptric microscope objective with a planarized image field. The microscope objective images an object at infinity, wherein the image is broadband, i.e. corrected over a large range of wavelengths. The collimated light beam from the microscope objective is then imaged with high magnification on a detector via a non-telecentric tube optical unit. In one variant, there is a variation of the imaging scale with an extension of approximately 3× (variation of the magnification from 36× to 100×) by changing the position of the detector (arranged in the non-telecentric beam path) and subsequent focusing. Consequently, this realizes a simple zoomable tube optical unit, in which, however, the system length of the overall system varies significantly depending on the set magnification.
In a further variant with the same extension (variation of the magnification from 36× to 100×), U.S. Pat. No. 7,136,159 B2 (Tsai et al.) shows a generic zoom system which images the collimated exit of the microscope objective onto a detector with zoomable imaging scale. What is shown here is a two-member zoom system or tele system, designed according to the teleobjective principle, with an object-side element group with positive refractive power and an image-side element group with negative refractive power, which vary the imaging scale. Here, a fixed installation length of the zoom system is achieved by displacing both element groups.
A problem here is that the Petzval sum, which is representative for the image field curvature, of such a tele system cannot be corrected; consequently, the tele system provides a strongly over-correcting contribution to the Petzval sum of the overall system. Here, this effect increases with an increasingly more compact design and/or with increasing magnification. However, in many cases it is desirable to design the optical interface between the microscope objective and the zoom system in an aberration-free and collimated manner, and so such a tele system, despite being able to achieve advantageously high magnifications, cannot in a simple manner realize the desired broadband freedom from aberration at a compact design.